1. Field of Invention
This invention relates to the commercial and industrial wastewater pretreatment field.
The invention is a mobile apparatus and batch pretreatment process for removing fats, oils and grease (commonly referred to as FOG) and suspended solids from wastewater traps, grease interceptors, septic tanks, pumping station wet wells, storage tanks and other related sanitary collection and storage facilities. The batch process steps include chemical addition, vacuum pumping into a flotation tank, vacuum flotation, clarification, sludge removal and sludge storage. The settled solids and FOG are concentrated to approximately five (5) percent solids during the process and transferred to an on board storage tank. The pre-treated liquid is returned to the source structure or discharged to the local sanitary sewer system.
2. Description of the Prior Art
Wastewater generated by food service establishments is required by most plumbing codes to be discharged to a grease interceptor prior to entering the sanitary sewer system. The interceptor captures fats, oils and grease (FOG) as well as large food particles that can cause blockage and maintenance problems in the sewer system. To function properly, grease interceptors must be routinely pumped to remove the FOG and settled solids. The normal practice is to empty the interceptor by vacuum pumping into a tanker truck and then haul the tank contents to a wastewater treatment facility or sanitary landfill as described in U.S. Pat. Nos. 4,975,195 and 5,543,050. Because of the expense to haul this wastewater, a need has arisen for an effective and efficient method to separate the FOG and settled solids at the grease interceptor location.
Likewise, FOG generated by households or establishments without a grease interceptor is discharged directly to a sewer system where it accumulates in sewer pipes, manholes and especially in wastewater pumping stations. Where households and businesses employ an on-site treatment system, the FOG is captured in a septic tank which acts similar to the grease interceptor and prevents the grease from plugging the drainfield. Each of these structures may require periodic pumping to remove the FOG and settled solids for which the proposed invention may be utilized. For the purposes of this discussion, all of the devices or structures mentioned where FOG accumulates will be referred to as grease interceptors.
A number of in-situ methods have been proposed including both biological and physical methods and apparatus for FOG removal; however, none anticipate the proposed invention and generally rely on either aeration to promote biological decomposition of the FOG or gravity separation and removal after a predetermined quantity of FOG has accumulated within the interceptor.
Various stationary processes are used at wastewater treatment facilities for treatment of hauled food service establishment sludge and most involve a screening step followed by blending with septage or sewage sludge prior to digestion or lime stabilization. Dissolved gas flotation is a common wastewater treatment method, especially dissolved air flotation, for the treatment of wastewaters containing large quantities of oils and grease. Most involve either injecting compressed gas directly into a flotation tank through a diffuser mechanism or saturating a pressurized recycle stream with the dissolved gas and injecting the pressurized fluid into the tank. These flotation tanks are normally under atmospheric pressure and once the compressed gas or pressurized fluid enters the tank, the reduced pressure causes the dissolved gas to come out of solution forming tiny size bubbles of less than 100 microns which attach to oil droplets and other suspended solids and float to the surface of the tank where they are removed by skimming devices. Dissolved gas flotation tanks are generally stationary vessels and operate in a continuous flow-through mode several of which are described in U.S. Pat. Nos. 4,216,085; 5,702,612; 5,766,484; and 6,599,418.
Chemical addition of hydrogen peroxide to the influent flow to a flotation unit is presented in U.S. Pat. No. 6,649,067 but this process relies on a chemical reaction to occur in the presence of a catalyst on the walls of the flotation tank which causes the decomposition of the hydrogen peroxide to oxygen and water.
A combined coarse and fine bubble system is presented in U.S. Pat. No. 5,275,732 wherein vacuum flotation is identified as one of several fine bubble methods to be used for flotation. This invention is a two step operation with a coarse gas bubble mixing step followed by a fine gas bubble flotation both of which utilize a bubble chamber and distribution device within the chamber for creating and dispersing the coarse and fine bubbles.
Vacuum flotation technology is referenced in Wastewater Engineering: Treatment, Disposal, Reuse, Second Edition by Metcalf & Eddy, Inc., copyright 1979, McGraw-Hill Book Company as an industrial process wherein the wastewater to be treated is first saturated with air either directly in an aeration tank or by induction into the waste stream during pumping into the flotation tank. The tank is described as a covered, cylindrical tank wherein a partial pressure is applied to cause the dissolved air to come out of solution and rise to the surface forming a scum blanket which is removed by a skimming mechanism.